I think your correct in that assumption. A PCM reciever will hold a position even though there is a problem, masking the fact that there is a problem inthe first place. Now if the problem is actually in the servo itself this theory wouldn't hold water. If there is an intermittent contact in any of the connection the servo(s) will twitch, even if they are being controlled by PCM as the problem is downstream, so to speak.

If you have any friends that have a spare Hitec receive you might give one of those a try and see if it makes a difference, couldn't hurt. I personally hate 127DF's as they have been the worst I have used when it came to unexplained glitching. Once you get the set up to prove to be quiet electrically then put the PCM back in the plane.